1. Field of the Invention
This invention relates to a coloring agent for use in toners, and a toner for developing electrostatic latent images or a toner for forming toner images in image-forming methods of a toner jet system. More particularly, this invention relates to a coloring agent the particle surfaces of which have been modified.
2. Related Background Art
Commonly available processes for producing toner particles of toners for developing electrostatic latent images and of toners used in image-forming methods of a toner jet system are known to include a pulverization process in which a binder resin and a coloring agent are melt-kneaded and the kneaded product is pulverized, and a polymerization process including dispersion polymerization such as suspension polymerization in which a fluid dispersion prepared by dispersing a coloring agent and so forth in a monomer or monomers for forming a binder resin is suspended in water to effect polymerization, or emulsion polymerization, and a process in which all constituents of toner particles, including a binder resin, are dissolved or dispersed in an organic solvent and the resultant solution or fluid dispersion is suspended in water followed by removal of the solvent to effect granulation.
In the above production processes, how the coloring agent be dispersed in toner particles in a good efficiency is an important item for improving the chroma, degree of pigmentation and transparency on OHT (overhead-projection transparent sheet) of toners, and various researches and proposals have been made thereon.
As one of processes for producing toner particles, a production process is known in which a mechanical or other method is used to incorporate a coloring agent into surface portions of toner particles comprised of at least a binder resin. In such toners in which the coloring agent is relatively much present at the surfaces of toner particles produced by such a production process or the pulverization process, it is considered difficult to attain a sufficient charge quantity in many cases.
To cope with such a problem, the following proposals have been made in order to overcome this problem by modifying particle surfaces of the coloring agent.
In Japanese Patent Application Laid-open No. 6-167833, a method is proposed in which a pigment is made to adsorb a copolymer of at least one monomer selected from a vinyl aromatic hydrocarbon and an acrylate or methacrylate monomer with sodium styrenesulfonate, in an attempt to improve the dispersibility of the pigment in the toner particles in a process for producing toner particles by polymerization.
In Japanese Patent No. 2859950, as a method of improving the dispersibility of a coloring agent in the pulverization process or the process in which a solution or fluid dispersion prepared by dissolving or dispersing a binder resin and other materials in an organic solvent is dispersed in water to effect granulation to obtain toner particles, a method is proposed in which a compound having at least one >C═N< linkage in the molecular is used as a dispersing agent of the coloring agent.
In Japanese Patent Application Laid-open No. 7-84410, as a method of improving the dispersibility of a coloring agent when toner particles are produced by using suspension polymerization, a method is proposed in which the coloring agent is dispersed in a polymerizable monomer together with a peroxide type polymerization initiator and a polymerization initiator is further added to effect suspension polymerization.
In Japanese Patent Publication No. 8-10354, a method is proposed in which a polymerization initiator is added to a fluid dispersion prepared by adding a coloring pigment to a polymerizable monomer containing an organic peroxide having an alkyl group having 13 or more carbon atoms, followed by suspension polymerization of this polymerizable monomer in an aqueous medium to produce toner particles.
These methods, however, are methods in which a fluid dispersion of a coloring agent is prepared or the particle surfaces of a coloring agent are modified in the course of the production of toner particles. Such methods are insufficient for achieving an object that sufficient charge quantity can be retained even where the coloring agent stands bare to the particle surfaces.
In Japanese Patent Application Laid-open No. 11-7163, a method is also proposed in which a pigment is encapsulated so as to improve the dispersibility of the pigment and also to put off any shortage of charge quantity which is due to the pigment coming bare to any broken surfaces in toner particles obtained by pulverization. However, the method of encapsulating the pigment proposed here, which is typified by a hetero-agglomeration method, requires a large number of steps or makes it necessary to control the pH delicately.
In the past, a method has been proposed in which azo groups or peroxide groups are chemically joined to particle surfaces of a coloring agent and a polymeric compound is grown therefrom as base points to coat the particle surfaces of the coloring agent with a resin. This method is effective where the coloring agent has highly reactive groups on its particle surfaces like carbon black. However, where the particle surfaces are inert or low-reactive like commonly available organic or inorganic coloring agents, it is usually difficult to make such groups join chemically to the particle surfaces of such coloring agents. Thus, it has been difficult to modify particle surfaces of coloring agents.